1. Field of the Invention
The invention relates to a circuit arrangement for producing an acoustic ringing signal in subscriber end stations of telecommunication arrangements in which the subscriber end stations are called from the exchange by superposing a ringing a.c. voltage on the d.c. supply voltage on the subscriber line.
2. Description of the Related Art
In telecommunication apparatus with alternating current bells, the ringing a.c. voltage which, in Germany, for example, may have a frequency of 25 Hz, is applied to the alternating current bell through a capacitor. When the hand-set is lifted by the subscriber, then by means of the hook switch the loop is closed over the subscriber line, the flow of the loop direct current is detected in the exchange, and the ringing alternating current is switched off.
In contemporary telephone sets, an electronic circuit is used to produce the acoustic ringing signal instead of the alternating current bell. The rectified ringing a.c. voltage is used to operate the electronic circuit. In the most simple case a rectifier is provided for this purpose, which is followed by a filter capacitor. The d.c. voltage thus generated is used to supply an electronic oscillator which in its turn controls a loud-speaker for sound reproduction.
Elektronik 1/15.01.1982, pages 53 to 58 "Microcomputer gesteuerter Tastwahl-Fernsprchapparat mit Flussigkristallanzeige", by Joachim Hollmann, discloses a telephone set with push button dial, comprising an electronic tone ringing circuit (tone ringing IC) for producing the acoustic ringing signal. The tone ringing-IC produces three frequencies: 800, 1067 and 1333 Hz, which are cyclically applied to a loudspeaker. The cyclic frequency and the volume can be adjusted by the subscriber. In addition to the tone ringing circuit the push-button dial telephone set comprises a dialing selection unit and a speech circuit. Control of the selection unit in accordance with the dialing information applied by the key board is provided by a microcomputer which also controls indicator arrangements and memory arrangements for storing call numbers or such like. If the dialing characters are produced in accordance with the multi-frequency method then the push button dial-telephone set comprises also an integrated circuit which is controlled by the key-board, and, each time a button is depressed, generates the two frequencies (line and row frequencies) which are transmitted to the exchange over the subscriber's line. The above-mentioned circuit arrangement always comprises separate IC's with associated quartz or ceramic resonator, respectively, for producing the frequencies of the multifrequency dialing method and for the acoustic ringing signal.
DE-AS No. 28 16 782 discloses a circuit arrangement for producing an acoustic ringing signal in subscriber stations of tele communication systems. The subscriber terminal includes a rectifier arrangement which, by rectifying the ringing alternating current applied to the subscribers line, generates the supply voltage for a switching arrangement (tone calling circuit), a tone-frequency oscillator (multivibrators) and an acoustic signal element (piezo-electric transcucer). So as to keep the power requirements for the circuit arrangement as low as possible, a piezo-electric transducer is connected by a switching device to either the supply potential or the ground reference potential potential. The frequencies produced by the multivibrator control the switching device, switching between two other frequencies being effected at a rate determined by one of the frequencies. This known circuit arrangement indeed requires only an inexpensive and simple circuit in the subscriber terminal but it has the disadvantage that the acoustic ringing signal may be triggered by noise voltages.
In the international application having the German file number P 3049811.4, corresponding to U.S. Pat. No. 4,276,448 an electronic tone ringer is disclosed which includes a sensor (tone calling circuit) for distinguishing between valid and invalid call signals and in which the tone calling circuit controls an oscillator-generator having a frequency-determining arrangement (tone-frequency oscillator) and an output circuit with electro-acoustic transducer (acoustic signal element). The tone calling circuit disclosed therein has the disadvantage that the circuit it requires in the subscribers terminal is very expensive and complicated.